Question

我正在尝试使用libvlc_media_new_callbacks函数使用libvlc从内存中播放媒体。此函数需要指向四个回调函数：Open，Read，Seek和Close。

Declare Function libvlc_media_new_callbacks Lib "libvlc.dll" (Instance As Ptr, OpenCB As Ptr, ReadCB As Ptr, SeekCB As Ptr, CloseCB As Ptr, Opaque As Ptr) As Ptr

Dim opaque As Ptr = SomeOpaqueValue()
Dim handle As Ptr = libvlc_media_new_callbacks(instance, AddressOf MediaOpen, AddressOf MediaRead, AddressOf MediaSeek, AddressOf MediaClose, opaque)

如果我没有指定Read回调，Seek，Close和Open回调都可以正常工作，但这意味着libvlc不会知道媒体是。

当我执行指定Open回调时，它会使用正确的opaque参数调用，但其他回调会获得空指针。这使得无法知道回调应该处理哪个流。

我是否误解了开放回调的目的？

以下是我的回调函数：

Shared Function MediaOpen(Opaque As Ptr, Buffer As Ptr, ByRef BufferSize As UInt64) As UInt32
  Dim r As BinaryStream = Streams.Lookup(Opaque, Nil)
  If r = Nil Then Return 1 ' invalid Opaque

  Buffer = Nil  ' Not sure what to do with this parameter
  BufferSize = r.Length
  Return 0
End Function

Shared Sub MediaClose(Opaque As Ptr)
  If Streams.HasKey(Opaque) Then Streams.Remove(Opaque)
  If Streams.Count = 0 Then Streams = Nil
End Sub

Shared Function MediaRead(Opaque As Ptr, Buffer As Ptr, BufferSize As Integer) As UInt32
  Dim r As BinaryStream = Streams.Lookup(Opaque, Nil)
  If r = Nil Then Return 0 ' invalid Opaque

  Dim mb As MemoryBlock = Buffer
  Dim data As MemoryBlock = r.Read(BufferSize)
  mb.StringValue(0, data.Size) = data
  Return data.Size
End Function

Shared Function MediaSeek(Opaque As Ptr, Offset As UInt64) As Int32
  Dim r As BinaryStream = Streams.Lookup(Opaque, Nil)
  If r = Nil Then Return 1 ' invalid Opaque
  If Offset > r.Length Then Return 1 ' invalid offset

  r.Position = Offset
  Return 0
End Function

Answer 1

根据文档，您必须分配一个缓冲区并在open的缓冲区参数中返回其地址（您需要将其更改为ByRef parm！）。然后你将获得在其他函数中传递的相同缓冲区ptr。

理想情况下，您需要设置

Buffer = r

MediaOpen中的

。但是Xojo不会让你失望。相反，您需要维护一个Dictionary，在其中存储r，然后将其与您可以分配给Buffer parm的值相关联。

这是一个例子（未经测试）：

Static Property OpenedBuffers as Dictionary
Static Property OpenedBufferID as Integer

Shared Function MediaOpen(Opaque As Ptr, ByRef HandleOut As Integer, ByRef BufferSize As UInt64) As UInt32
  Dim r As BinaryStream = Streams.Lookup(Opaque, Nil)
  If r = Nil Then Return 1 ' invalid Opaque
  if OpenedBuffers = nil then OpenedBuffers = new Dictionary
  OpenedBufferID = OpenedBufferID + 1
  OpenedBuffers.Value(OpenedBufferID) = r
  HandleOut = OpenedBufferID
  BufferSize = r.Length
  Return 0
End Function

Shared Function MediaRead(Handle As Integer, Buffer As Ptr, BufferSize As Integer) As UInt32
  Dim r As BinaryStream = OpenedBuffers.Value(Handle)
  ...

Shared Sub MediaClose(Handle As Integer)
  Dim r As BinaryStream = OpenedBuffers.Value(Handle)
  OpenedBuffers.Remove (Handle)
  If Streams.HasKey(r) Then Streams.Remove(r)
  ...

希望有所帮助

正确使用libvlc_media_open_cb

1 个答案: